1. Field of the Invention
This invention relates to the bag making art and more particularly the preferred embodiment relates to a short dwell indexing drive system for intermittently moving a web of bag making material smoothly through the bag machine with the web advancing or indexing movement of each cycle being in excess of 180.degree. of the cycle.
2. Description of the Prior Art
Bag making machines are well known in the art as evidenced by my U.S. Pat. No. 3,663,338 which issued on May 16, 1972 and which is incorporated by reference herein. In this prior art machine a web of thermosealing bag making material is engaged between draw rolls which are intermittently rotated to drive the web one bag length or width for each cycle of operation. The drive for the draw rolls comprises a continuously driven crank shaft which makes one complete revolution for each bag making cycle. A crank pin on the crank shaft is connected by a crank arm to a gear segment which moves forwardly through 180.degree. of each cycle and rearwardly through the remaining 180.degree. of each bag making cycle. A clutch-brake assembly is included in a drive train which connects the gear segment to the draw rolls. During the forward moving half cycle of the gear segment, the clutch is engaged and the brake is disengaged to drive the web forwardly in simple harmonic motion with the maximum web velocity being about 940 feet per minute for making 30 inch bags at the rate of 120 bags per minute. During the rearward moving half cycle of the gear segment the clutch is released and the brake is engaged thus holding the draw rolls stationary during a dwell period which is one-half of the bag making cycle. During the dwell a transverse heat seal and a severance operation takes place to permit separation of a completed bag from the web.
U.S. Pat. No. 2,997,889 which issued to Schjeldahl et al. on Aug. 29, 1961 discloses a similar draw roll drive which imparts harmonic motion to the web during only 180.degree. of each cycle. In this machine a crank oscillates a drive shaft by means of a rack and pinion drive. A drive train from the oscillating drive shaft to the draw rolls includes a clutch-brake assembly which operates to drive or index the web forwardly through one half of the cycle of the drive shaft and to dwell through the other half of the cycle at which time transverse sealing is performed and the bag is severed from the web.
U.S. Pat. No. 3,776,804 which issued to Monahan et al. on Dec. 4, 1973 discloses a short dwell draw roll drive system which includes a phased harmonic drive. The phased harmonic drive includes a pair of cranks on a continuously driven input or crank shaft having their crank pins angularly offset or out of phase with each other. Each crank pin is connected to a rack which rotates a pinion connected to a stub shaft thereby oscillating the associated stub shaft. Each stub shaft is connected to a draw roll through a clutch brake assembly that is disposed between the associated stub shaft and the draw roll. When the first stub shaft is at zero velocity just prior to forward movement, the clutch of its clutch-brake assembly is engaged and remains engaged for a period in excess of 90.degree. thus accelerating the draw roll drive shaft to its peak velocity at 90.degree. in a forward direction and thereafter decelerating the drive shaft until its decelerating harmonic curve intersects the accelerating portion of the harmonic motion curve of the second stub shaft that is driven by the out of phase second crank. At this point and only this point of intersection of the two curves, either stub shaft, if clutched to the draw roll, would drive the web at the same speed. At this critical point, the first clutch is disengaged and the clutch of the second clutch-brake assembly is engaged to connect the second stub shaft to the draw roll. With the second clutch engaged, the draw roll again accelerates in harmonic motion to its peak velocity and thereafter decreases in velocity to zero velocity at a point somewhat beyond the 180.degree. mark of the bag making cycle. When reaching the zero velocity point, the second clutch is disengaged and the brake is engaged to hold the drive shaft and draw roll in a dwell throughout the remainder of the bag making cycle at which time the bag making cycle is repeated for the next bag. Thus, the double harmonic drive provides a web feed or indexing motion which is in excess of 180.degree. and a dwell that is less than 180.degree. of the bag making cycle. However, the feed imparts two accelerating and decelerating motions to each bag with each bag moving through two high velocity peaks.
U.S. Pat. Nos. 2,986,949 which issued to Lancaster et al on June 6, 1961 and U.S. Pat. No. 3,525,268 which issued to Kenny on Aug. 25, 1970 discloses conjugate cam drives of the type employed by applicant in the indexing drive system of the present invention. The cam drives include a driven cam shaft having a pair of cams thereon. Each of the cams is in planar alignment with one of two series of equally spaced cam followers secured to an output shaft. The contour of the cams are designed so that both cams are at all times in firm engagement with one of the cam followers of its series thus eliminating substantially all backlash of the output shaft. The specific contour of the two cams, and the number of cam followers in each series determines the proportion of each cycle of the output shaft used for dwell and the motion characteristics of the portion of each cycle used for indexing. Since conjugate cam drives of this type are incorporated in the indexing roll drive system of the present invention, the disclosure of these patents are included by reference herein.